Hitherto, as this type of assist device, there have been known one shown in FIG. 9 of Japanese Patent Application Publication No. 2001-198864 (hereinafter referred to as Patent Document 1) and one disclosed in Japanese Patent Application Publication No. 2003-145477 (hereinafter referred to as Patent Document 2) and one disclosed in Japanese Patent Application Publication No. 2003-103480 (hereinafter referred to as Patent Document 3) previously proposed by the present applicant.
The one disclosed in Patent Document 1 includes a spring provided between two link members (a thigh and a crus) connected by a knee joint of each leg of a bipedal mobile robot, a flexing or stretching motion of each leg at the knee joint being converted into an elastic energy of the spring. When the robot jumps, an assisting driving force (elastic force) is applied to the knee joint by the elastic energy. In this case, if the flexing angle of each leg at the knee joint is a predetermined angle, then the spring will be in a natural length state (a state wherein the elastic energy has been released), while the elastic energy will be accumulated in the spring at a flexing angle other than the predetermined angle, thus producing an assisting driving force.
The one disclosed in Patent Document 2 is adapted to convert a flexing or stretching motion of each leg at the knee joint into the elastic energy of a gas spring composed of a cylinder filled with a gas and to apply the assisting driving force (elastic force) to the knee joint by the elastic energy. In this case, the operational characteristics of the gas spring (the kinetic characteristics of a piston of the gas spring in response to flexing/stretching motions of each leg) are set such that, during a supporting leg period, as the flexing angle of the supporting leg at the knee joint (the inclination angle of the axis of the crus relative to the axis of the thigh) increases, the assisting driving force generated by the gas spring increases in the direction in which the leg stretches, and the assisting driving force (the assisting driving force in the stretching direction) generated by the gas spring reaches substantially a maximum value when the flexing angle of each leg at the knee joint reaches substantially a maximum (maximal) value. With this arrangement, a proper assisting driving force is generated by the gas spring in the supporting leg period during which a driving force to be generated at the knee joint reaches its maximum value, thus reducing a burden on a joint actuator of the knee joint.
The one disclosed in Patent Document 3 is provided with a spring means (a gas spring or the like) that converts flexing/stretching motions of each leg at the knee joint into elastic energy, and also provided with a mechanism for cutting off the transmission of the flexing/stretching motions of each leg to the spring means, as appropriate (hereinafter referred to as the locking mechanism). In this case, according to the one disclosed in Patent Document 3, the locking mechanism is actuated at a predetermined timing through the intermediary of an electromagnetic solenoid or a solenoid valve so as to generate the assisting driving force (elastic force) in the spring means in a mode wherein the flexing/stretching motions of the leg are transmitted to the spring means during a period in which the assisting driving force should be imparted to the knee joint. In a period other than that, a mode wherein the flexing/stretching motions of the leg are not transmitted to the spring means is set so as to prevent the assisting driving force (elastic force) from being imparted to the knee joint from the spring means.
In the one disclosed in Patent Document 1, the assisting driving force by the spring is not generated only when the flexing angle of the leg at the knee joint is the predetermined angle, meaning that the elastic force by the spring is always generated at any other flexing angles. For this reason, even if a required driving force to be generated at the knee joint of each leg is small, there are cases where a driving force for canceling the elastic force of the spring must be produced in the joint actuator of the knee joint, resulting in an increase in the energy consumed by the joint actuator in some cases. Especially in the one disclosed in Patent Document 1, the elastic force of the spring increases as the flexing angle of the leg at the knee joint increases, so that a driving force generated at the joint actuator in a free leg period of a leg, during which the required driving force to be generated at the knee joint should be small, will be inevitably large, thus making it difficult to restrain a generated driving force of the joint actuator and eventually energy consumption.
Further, the aforesaid one disclosed in Patent Document 2 makes it possible to minimize the maximum value of the generated driving force of the joint actuator over the entire period when the robot travels, permitting a reduced burden on the joint actuator to be achieved. However, the flexing angle of the knee joint at which the elastic force of the gas spring is zero is limited to a certain particular angle, and the elastic force of the gas spring is generated in the leg flexing direction in a state wherein the flexing angle is small (in a state wherein the leg is substantially stretched). Hence, a driving force for canceling the elastic force of the gas spring must be generated in the joint actuator of the knee joint in a situation wherein the driving force to be generated in the knee joint is the driving force in the leg stretching direction, as in a state wherein the robot is standing upright, or in a situation wherein the driving force to be generated in the knee joint is substantially zero, as in a state wherein a supporting leg is about to leave a floor when a robot travels. Thus, the one disclosed in Patent Document 2 has been inadequate for accomplishing a further reduction in the energy consumption of the joint actuator of the knee joint.
In the aforesaid one disclosed in Patent Document 3, the elastic force (assisting driving force) of the spring means can be imparted to the knee joint only when necessary; however, the aforesaid locking mechanism and an electromagnetic solenoid or a solenoid valve for operating the locking mechanism are required, inconveniently leading to a larger or a complicated construction. Furthermore, energy for operating the electromagnetic solenoid or the solenoid valve is required, hindering a further reduction in the energy consumption of the robot.
The present invention has been made with a view of the background described above, and it is an object of the invention to provide a leg joint assist device that permits a reduction in energy consumption of a robot and also a reduction in a burden on a joint actuator of a leg by using a small and simple construction.